In the field of light-emitting diodes (LED), it is well known to use phosphors for wavelength conversion (Patent Document 1: JP-A 2005-524737). On the other hand, as the coating material for encapsulating and protecting LED chips, silicone resins are regarded attractive because of their light resistance (Patent Document 2: JP-A 2004-339482).
White LEDs are generally constructed by coating a blue-emitting LED chip with a silicone or epoxy resin compound having a phosphor dispersed therein so that the phosphor is distributed in proximity to the chip for converting blue light to pseudo-white light.
However, if the distribution of the phosphor within the resin layer is non-uniform or graded, then a color shift is likely to occur. The phosphor must be uniformly dispersed within the resin layer before uniform white light can be produced. In order to achieve such uniform dispersion, studies are made on screen printing and phosphor dispersion via precipitation. These methods still have some problems including complex production steps and inadequate stability. Accordingly, a simple technique capable of uniformly dispersing a phosphor over a chip surface is sought after. In this context, Patent Document 3 (JP 4927019) proposes to encapsulate LED chips with a normally solid or semisolid, addition curable silicone resin having a phosphor uniformly dispersed therein.
As advanced LEDs become of higher power, the resin layer for coating the LED chip is required to have better heat resistance and UV resistance. Even the addition curable silicone resin of Patent Document 3 fails to clear the level of heat resistance and UV resistance. This is because hydrosilylation catalysts such as platinum complexes, palladium complexes and rhodium complexes are not only essential for addition curable silicone resins to cure, but also act to accelerate degradation of the resins. It is thus necessary to minimize the amount of the hydrosilylation catalyst while maintaining a cure rate so as to avoid any drop of working efficiency and productivity.
As the means for minimizing the amount of the hydrosilylation catalyst while maintaining curability, a combination of addition cure and condensation cure is contemplated effective. For the condensation cure that generally proceeds at a slow cure rate, condensation catalysts must be used. However, these condensation catalysts also cause to degrade the silicone resins, giving rise to the problem that the resins lose heat resistance and light resistance. Some catalysts themselves are colored, or the resins are colored due to degradation. Many catalysts are inadequate in the application where transparency is important.